From: John 17:20-26
The Priestly Prayer of Jesus (Continuation)
(Jesus lifted His eyes to Heaven and said,)  "I do not pray for these (My disciples) only, but also for those who believe in Me through their word,  that they may all be one; even as Thou, Father, art in Me, and I in Thee, that they also may be in us, so that the world may believe that Thou hast sent Me.  The glory which Thou hast given Me I have given to them, that they may be one even as we are one,  I in them and Thou in Me, that they may become perfectly one, so that the world may know that Thou hast sent Me and hast loved them even as Thou hast loved Me.  Father, I desire that they also, whom Thou hast given Me, may be with Me where I am, to behold My glory which Thou hast given Me in Thy love for Me before the foundation of the world.  O righteous Father, the world has not known Thee, but I have known Thee; and these know that Thou hast sent Me.  I made known to them Thy name, and I will make it known, that the love with which Thou hast loved Me may be in them, and I in them."
20-23. Since it is Christ who is praying for the Church His prayer is infallibly effective, and therefore there will always be only one true Church of Jesus Christ. Unity is therefore an essential property of the Church. "We believe that the Church founded by Jesus Christ and for which He prayed is indefectibly one in faith, in worship and in the bond of hierarchical communion" ([Pope] Paul VI, "Creed of the People of God", 21). Moreover, Christ's prayer also indicates what the basis of the Church's unity will be and what effects will follow from it.
The source from which the unity of the Church flows is the intimate unity of the Three Divine Persons among whom there is mutual love and self-giving. "The Lord Jesus, when praying to the Father `that they may all be one...even as we are one' (John 17:21-22), has opened up new horizons closed to human reason by implying that there is a certain parallel between the union existing among the Divine Persons and the union of the sons of God in truth and love. It follows, then, that if man is the only creature on earth that God has wanted for its own sake, man can fully discover his true self only in a sincere giving of himself" (Vatican II, "Gaudium Et Spes", 24). The unity of the Church is also grounded on the union of the faithful with Jesus Christ and through Him with the Father (verse 23). Thus, the fullness of unity--"consummati in unum"--is attained through the supernatural grace which comes to us from Christ (cf. John 15:5).
The fruits of the unity of the Church are, on the one hand, the world believing in Christ and in His divine mission (verses 21, 23); and, on the other hand, Christians themselves and all men recognizing God's special love for His faithful, a love which is a reflection of the love of the Three Divine Persons for each other. And so, Jesus' prayer embraces all mankind, for all are invited to be friends of God (cf. 1 Timothy 2:4). "Thou hast loved them even as Thou hast loved Me": this, according to St. Thomas Aquinas, "does not mean strict equality of love but similarity and like-motivation. It is as if He were saying: the love with which You have loved Me is the reason and the cause of Your loving them, for, precisely because You love men do You love those who love Me" ("Commentary on St. John, in loc."). Besides noting this theological explanation, we should also ponder on how expressively Christ describes His ardent love for men. The entire discourse of the Last Supper gives us a glimpse of the depth of Jesus' feelings--which infinitely exceeds anything we are capable of experiencing. Once again all we can do is bow down before the mystery of God-made-man.
20. Christ prays for the Church, for all those who, over the course of centuries, will believe in Him through the preaching of the Apostles. "That divine mission, which was committed by Christ to the Apostles, is destined to last until the end of the world (cf. Matthew 28:20), since the Gospel, which they were charged to hand on, is, for the Church, the principle of all its life for all time. For that very reason the Apostles were careful to appoint successors in this hierarchically constituted society" (Vatican II, "Lumen Gentium", 20).
The apostolic origin and basis of the Church is what is termed its "apostolicity", a special characteristic of the Church which we confess in the Creed. Apostolicity consists in the Pope and the Bishops being successors of Peter and the Apostles, holding the authority of theApostles and proclaiming the same teaching as they did. "The sacred synod teached that the bishops have by divine institution taken the place of the Apostles as pastors of the Church, in such wise that whoever listens to them is listening to Christ and whoever despises them despises Christ and Him who sent Christ (cf. Luke 10:15)" (Vatican II, "Lumen Gentium", 20).
21. Union of Christians with Christ begets unity among themselves. This unity of the Church ultimately redounds to the benefit of all mankind, because since the Church is one and unique, she is seen as a sign raised up for the nations to see, inviting all to believe in Christ as sent by God come to save all men. The Church carries on this mission of salvation through its union with Christ, calling all mankind to join the Church and by so doing to share in union with Christ and the Father.
The Second Vatican Council, speaking of the principles of ecumenism, links the Church's unity with her universality: "Almost everyone, though in different ways, longs for the one visible Church of God, a Church truly universal and sent forth to the whole world that the world may be converted to the Gospel and so be saved, to the glory of God" ("Unitatis Redintegratio", 1). This universality is another characteristic of the Church, technically described as "catholicity". "For many centuries now the Church has been spread throughout the world, and it numbers persons of all races and walks of life. But the universality of the Church does not depend on its geographical distribution, even though this is a visible sign and of motive of credibility. The Church was catholic already at Pentecost: it was born catholic from the wounded heart of Jesus, as a fire which the Holy Spirit enkindled.
"In the second century the Christians called the Church catholic in order to distinguish it from sects, which, using the name of Christ, were betraying His doctrine in one way or another. `We call it catholic', writes St. Cyril, `not only because it is spread throughout the world, from one extreme to the other, but because in a universal way and without defect it teaches all the dogmas which men ought to know, of both the visible and the invisible, the celestial and the earthly. Likewise because it draws to true worship all types of men, governors and citizens, the learned and the ignorant. And finally, because it cures and heals all kinds of sins, whether of the soul or of the body, possessing in addition--by whatever name it may be called--all the forms of virtue, in deeds and in words and in every kind of spiritual life' ("Catechesis", 18, 23)" ([St] J. Escriva, "In Love with the Church", 9).
Every Christian should have the same desire for this unity as Jesus Christ expresses in His prayer to the Father. "A privileged instrument for participation in pursuit of the unity of all Christians is prayer. Jesus Christ Himself left us His final wish for unity through prayer to the Father: `that they also may be in us, so that the world may believe that Thou hast sent Me' (John 17:21).
"Also the Second Vatican Council strongly recommended to us prayer for the unity of Christians, defining it `the soul of the whole ecumenical movement' ("Unitatis Redintegratio", 8). As the soul to the body, so prayer gives life, consistency, spirit, and finality to the ecumenical movement.
"Prayer puts us, first and foremost, before the Lord, purifies us in intentions, in sentiments, in our heart, and produces that `interior conversion', without which there is no real ecumenism. (cf. "Unitatis Redintegratio", 7).
"Prayer, furthermore, reminds us that unity, ultimately, is a gift from God, a gift for which we must ask and for which we must prepare in order that we may be granted it" ([Pope] John Paul II, "General Audience", 17 January 1979).
22-23. Jesus possess glory, a manifestation of divinity, because He is God, equal to the Father (cf. note on John 17:1-5). When He says that He is giving His disciples this glory, He is indicating that through grace He makes us partakers of the divine nature (2 Peter 1:4). Glory and justification by grace are very closely united, as we can see from Sacred Scripture: "Those whom He predestined He also called, and those whom He called He also justified, and those whom He justified He also glorified" (Romans 8:30). The change grace works in Christians makes us ever more like Christ, who is the likeness of the Father (cf. 2 Corinthians 4:4; Hebrews 1:2-3): by communicating His glory Christ joins the faithful to God by giving them a share in supernatural life, which is the source of the holiness of Christians and of the Church: "Now we can understand better how [...] one of the principal aspects of her holiness is that unity centered on the mystery of the one and triune God. `There is one body and one Spirit, just as you were called to the one hope that belongs to your call, one Lord, one faith, one baptism; one God and Father of us all, who is above all and through all and in all' (Ephesians 4:4-6)" ([St] J. Escriva, "In Love with the Church", 5).
24. Jesus concludes His prayer by asking that all Christians attain the blessedness of Heaven. The word He uses, "I desire", not "I pray", indicates that He is asking for the most important thing of all, for what His Father wants--that all may be saved and come to a knowledge of the truth (cf. 1 Timothy 2:4): which is essentially the mission of the Church--the salvation of souls.
As long as we are on earth we share in God's life through knowledge (faith) and love (charity); but only in Heaven will we attain the fullness of this supernatural life, when we see God as He is (cf. 1 John 3:2), face to face (cf. 1 Corinthians 13:9-12). Therefore, the Church has her sights fixed on eternity, she is eschatological: that is, by having in this world all the resources necessary for teaching God's truth, for rendering Him true worship and communicating the life of grace, she keeps alive people's hope of attaining the fullness of eternal life: "The Church, to which we are all called in Christ Jesus, and in which by the grace of God we acquire holiness, will receive its perfection only in the glory of Heaven, when will come the time of the renewal of all things (Acts 3:21). At that time, together with the human race, the universe itself, which is so closely related to man and which attains its destiny through him, will be perfectly reestablished in Christ (cf. Ephesians 1:10; Colossians 1:20; 2 Peter 3:10-13)" (Vatican II, "Lumen Gentium", 48).
25-26. God's revelation of Himself through Christ causes us to begin to share in the divine life, a sharing which will reach its climax in Heaven: "God alone can give us right and full knowledge of this reality by revealing Himself as Father, Son and Holy Spirit, in whose eternal life we are by grace called to share, here below in the obscurity of faith and after death in eternal light" ([Pope] Paul VI, "Creed of the People of God").
Christ has revealed to us all we need to know in order to participate in the mutual love of the Divine Persons--primarily, the mystery of who He is and what His mission is and, with that, the mystery of God Himself ("I made known to them Thy name"), thus fulfilling what He had announced: "No one knows the Father except the Son and any one to whom the Son chooses to reveal Him" (Matthew 11:27).
Christ continues to make known His Father's love, by means of the Church, in which He is always present: "I am with you always, to the close of the age" (Matthew 28:20).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.